Robust Calibration and Self-Correction for Tissue Oximetry Probe

ABSTRACT

A method for calibrating detectors of a self-contained, tissue oximetry device includes emitting light from a light source into a tissue phantom, detecting in a plurality of detectors the light emitted from the light source, subsequent to reflection from the tissue phantom, and generating a set of detector responses by the plurality of detectors based on detecting the light emitted from the light source. The method further includes determining a set of differences between the set of detector responses and a reflectance curve for the tissue phantom, and generating a set of calibration functions based on the set of differences. Each calibration function in the set of calibration functions is associated with a unique, light source-detector pair. The method further includes storing the set of calibration function in a memory of the self-contained, tissue oximetry device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 15/359,570, filed Nov. 22, 2016, issued as U.S. Pat. No. 10,492,715on Dec. 3, 2019, which is a continuation of U.S. patent application Ser.No. 13/887,152, filed May 3, 2013, issued as U.S. Pat. No. 9,458,157 onNov. 22, 2016, which claims the benefit of U.S. patent applications61/642,389, 61/642,393, 61/642,395, and 61/642,399, filed May 3, 2012,and 61/682,146, filed Aug. 10, 2012. These applications are incorporatedby reference along with all other references cited in this application.

BACKGROUND OF THE INVENTION

The present invention relates generally to optical systems that monitoroxygen levels in tissue. More specifically, the present inventionrelates to oximeters that include sources and detectors on sensor headsfor emitting and detecting light.

Oximeters are medical devices used to measure oxygen saturation oftissue in humans and living things for various purposes. For example,oximeters are used for medical and diagnostic purposes in hospitals andother medical facilities (e.g., surgery, patient monitoring, orambulance or other mobile monitoring for, e.g., hypoxia); sports andathletics purposes at a sports arena (e.g., professional athletemonitoring); personal or at-home monitoring of individuals (e.g.,general health monitoring, or personal training for a marathon); andveterinary purposes (e.g., animal monitoring).

Pulse oximeters and tissue oximeters are two types of oximeters thatoperate on different principles. A pulse oximeter requires a pulse inorder to function. A pulse oximeter typically measures the absorbance oflight due to the pulsing arterial blood. In contrast, a tissue oximeterdoes not require a pulse in order to function, and can be used to makeoxygen saturation measurements of a tissue flap that has beendisconnected from a blood supply.

Human tissue, as an example, includes a variety of molecules that caninteract with light via scattering or absorption (e.g., vialight-absorbing chromophores). Such chromophores include oxygenated anddeoxygenated hemoglobins, melanin, water, lipid, and cytochrome.Oxygenated and deoxygenated hemoglobins are the most dominantchromophores in the spectrum range of 600 nanometers to 900 nanometers.Light absorption differs significantly for oxygenated and deoxygenatedhemoglobins at certain wavelengths of light. Tissue oximeters canmeasure oxygen levels in human tissue by exploiting theselight-absorption differences.

Despite the success of existing oximeters, there is a continuing desireto improve oximeters by, for example, improving measurement accuracy;reducing measurement time; lowering cost; reducing size, weight, or formfactor; reducing power consumption; and for other reasons, and anycombination of these.

In particular, assessing a patient's oxygenation state is important asit is an indicator of the state of the patient's health. Thus, oximetersare often used in clinical settings, such as during surgery andrecovery, where it may be suspected that the patient's tissueoxygenation state is unstable. For example, during surgery, oximetersshould be able to quickly deliver accurate oxygen saturationmeasurements under a variety of non-ideal conditions. While existingoximeters have been sufficient for post-operative tissue monitoringwhere speed of measurement is less critical, existing oximetersfluctuate substantially and give inaccurate saturation measurements whenused during surgery where various elements can interfere with accuratereading, such as if the oximeter comes in contact with blood.

Therefore, there is a need for improved oximeters and methods of makingmeasurements using these oximeters.

BRIEF SUMMARY OF THE INVENTION

A sensor head for a compact oximeter sensor probe includes light sourcesand light detectors. A probe implementation is entirely self-contained,without any need to connect, via wires or wirelessly, to a separatesystem unit. The sources and detectors probe are arranged in a circulararrangement having various source-detector pair distances that allow forrobust calibration and self-correction in a compact probe. Othersource-detector arrangements are also possible.

According to a specific embodiment, a sensor head for a tissue oximetrydevice includes at least a first light source and a second light forgenerating and emitting light into tissue, and a set of detectors fordetecting the light subsequent to reflection from the tissue. First andsecond detectors included in the set of detectors are positionedapproximately 1.5 millimeters or closer to the first light source or thesecond light source, or both. Third and fourth detectors included in theset of detectors are positioned approximately 2.5 millimeters or fartherfrom the first light source or the second light source, or both.

According to another specific embodiment, a sensor head for a tissueoximetry device includes a set of detectors positioned in a circulararrangement; and first and second light sources linearly positioned on abisecting line of a circle of the circular arrangement. A first detectorincluded in the set of detectors is nearest to the first light sourcerelative to all other detectors in the set of detectors and is a firstdistance from the first light source. A second detector included in theset of detectors is nearest to the second light source relative to theall other detectors in the set of detectors and is a second distancefrom the second light source. The first distance and the second distanceare equal.

According to another specific embodiment, a method for calibratingdetectors of a tissue oximetry device includes emitting light from alight source into a tissue phantom, and detecting in a plurality ofdetectors the light emitted from the light source, subsequent toreflection from the tissue phantom. The method further includesgenerating a set of detector responses by the plurality of detectorsbased on detecting the light emitted from the light source, andcomparing the set of detector responses with a reflectance curve for thetissue phantom. The method further includes generating a set ofcalibration functions based on the comparison. Each calibration functionin the set of calibration functions is associated with a unique, firstlight source-detector pair. The method further includes storing the setof calibration functions in a memory of the tissue oximetry device. Thesteps of this specific embodiment may be repeated for one or moreadditional light sources of the tissue oximeter, and may be repeated forone or more additional tissue phantoms.

According to another specific embodiment, a method for calibratingdetectors of a tissue oximetry device includes emitting light from alight source into a tissue phantom, wherein the light source isequidistance from a plurality of detectors; and detecting the light,which is reflected from the tissue phantom, in the plurality ofdetectors. The method also includes generating detector responses byeach detector in the plurality of detectors based on detecting thelight, which is reflected from the tissue phantom; and determiningdissimilarities between the detector responses. The method also includesgenerating calibration functions based on the dissimilarities, whereinif the calibration functions are applied to the raw detector responses,the dissimilarities of the detector responses are equalized.

According to another specific embodiment, a method for operating asensor head of a tissue oximetry device includes emitting light from alight source into tissue, wherein the light source is equidistance froma plurality of detectors; and detecting the light, which is reflectedfrom the tissue, in the plurality of detectors. The method also includesgenerating detector responses by each detector in the plurality ofdetectors based on detecting the light, which is reflected from thetissue. The method also includes determining whether at least one of thedetector responses for one of the detectors differs from the detectorsresponse of others of the detectors by at least a threshold; anddisregarding the at least one of the detector responses if the at leastone of the detector responses for the one of the detectors differs fromthe detectors response of the others of the detectors by at least thethreshold.

According to another specific embodiment, a method for calibrating thelight sources of a tissue oximetry device includes emitting light from afirst light source into tissue; and detecting in a first detector thelight emitted by the first light source subsequent to reflection fromthe tissue. The first detector is a first distance from the first lightsource. The method also includes generating a first detector responsebased on detecting the light in the first detector. The method alsoincludes emitting light from a second light source into the tissue; anddetecting in a second detector the light emitted by the second lightsource subsequent to reflection from the tissue. The second detector isa second distance from the second light source, and the first distanceand the second distance are equal. The method also includes generating asecond detector response based on detecting in the second detector thelight emitted by the second light source; and generating a calibrationfunction that represents a dissimilarity between the first detectorresponse and the second detector response if the first detector responseand the second detector response are not equal.

The above described embodiments of the tissue oximetry device and themethod of use enable robust detector calibration and provides foridentification of local inhomogeneity in real tissue and for discardingreflectance data from moles or other tissue aberrations. The positioningof the detectors with respect to the light sources provides a relativelylarge number of unique source-to-detector distances that increase theprobability that derived optical properties are accurate by decreasingreflectance data redundancy and also enables a fast, reliable correctionfor dissimilarities in the data caused by power differences betweensources.

In an implementation, the device is a tissue oximeter, which can measureoxygen saturation without requiring a pulse or heart beat. A tissueoximeter of the invention is applicable to many areas of medicine andsurgery including plastic surgery. The tissue oximeter can make oxygensaturation measurements of tissue where there is no pulse; such tissue,for example, may have been separated from the body (e.g., a flap) andwill be transplanted to another place in the body.

Aspects of the invention may also applicable to a pulse oximeter. Incontrast to a tissue oximeter, a pulse oximeter requires a pulse inorder to function. A pulse oximeter typically measures the absorbance oflight due to the pulsing arterial blood.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand the accompanying drawings, in which like reference designationsrepresent like features throughout the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a tissue oximetry device according to oneembodiment.

FIG. 1B is a simplified block diagram of the tissue oximetry device.

FIGS. 2A, 2B, and 2C are simplified end views of the tissue oximetrydevice.

FIGS. 3A and 3B are a simplified perspective view and a cross-sectionalview of the tissue oximetry probe, respectively.

FIG. 4 is a simplified diagram of the front PCB with the detectorspositioned in a circular arrangement on the PCB.

FIGS. 5A and 5B are a simplified perspective view and an exploded view,respectively, of a tissue oximetry probe according to another specificembodiment.

FIG. 6 is a high-level flow diagram of a method for calibrating eachsource-detector pair according to one embodiment.

FIG. 7 is a high-level flow diagram for a method for calibrating thedetectors according to one embodiment.

FIG. 8 is a high-level flow diagram for a method for detecting anomaliesduring use of the tissue oximetry device.

FIG. 9 is a high-level flow diagram for a method for calibrating theamount of light emitted by the outer sources during oxygen saturationmeasurements on tissue or with a tissue phantom.

DETAILED DESCRIPTION OF INVENTION

Spectroscopy has been used for noninvasive measurement of variousphysiological properties in animal and human subjects. Visible light(e.g., red) and near-infrared spectroscopy is often utilized becausephysiological tissues have relatively low scattering in this spectralrange. Human tissues, for example, include numerous chromophores such asoxygenated hemoglobin, deoxygenated hemoglobin, melanin, water, lipid,and cytochrome. The hemoglobins are the dominant chromophores in tissuefor much of the visible and near-infrared spectral range. In the 700-900nanometer range, oxygenated and deoxygenated hemoglobin have verydifferent absorption features. Accordingly, visible and near-infraredspectroscopy has been applied to measure oxygen levels in physiologicalmedia, such as tissue hemoglobin oxygen saturation and total hemoglobinconcentrations.

Various techniques have been developed for visible and near-infraredspectroscopy, such as time-resolved spectroscopy (TRS), frequency-domaintechniques such as phase modulation spectroscopy (PMS), and continuouswave spectroscopy (CWS). In a homogeneous and semi-infinite model ofphysiological media, both TRS and PMS have been used to obtain theabsorption coefficient and reduced scattering coefficient of the mediumby use of the photon diffusion approximation or Monte Carlo models. Fromthe absorption coefficients at multiple wavelengths, concentrations ofoxygenated and deoxygenated hemoglobins can be determined and the tissueoxygen saturation calculated. CWS generally does not possess enoughinformation to separate the effects of scattering and absorption. It hastypically been used to solve a modified Beer-Lambert equation thatrequires assumptions about tissue scattering and two or more wavelengthsare used ratiometrically to cancel out optical pathlength, which wouldotherwise be required to solve the equation. CWS, in its commonly-usedform, provides relative oxygen saturation only and cannot provideabsolute oxygen saturation or concentrations of oxygenated anddeoxygenated hemoglobins.

Despite their capability of providing hemoglobin concentrations andabsolute oxygen saturation, one major drawback of TRS and PMS is thatthe equipment is bulky and expensive. Another major drawback is thatboth of these techniques have difficulty measuring through relativelysmall volumes of tissue (i.e., “local” measurement, within a fewmillimeters). These techniques are typically used for “regional”measurements (minimum of 1 centimeter) due to the small time changes orphase shifts associated with short transit time through small volumes oftissue. In contrast, CWS may be manufactured at a lower cost, but istypically limited in its utility as described above unless enhancementsare made by either including broadband spectral information or byincluding spatial information.

Spatially resolved spectroscopy (SRS) is one type of near-infraredspectroscopy that allows tissue absorption to be determinedindependently from tissue scattering, thereby allowing absolutemeasurements of chromophore concentrations.

More specifically, an SRS instrument emits light into tissue through asource and collects the diffusely reflected light at two or moredetectors at different distances from the source. Alternatively, lightmay be emitted from two or more sources at different distances to one ormore detectors. Scattering of light back to the detector is caused byrelative changes in index of refraction of the tissue and includes Miescattering from larger structures such as mitochondria (the majority oftissue scattering is a result of mitochondria) and Rayleigh scatteringfrom smaller structures such as intracellular vesicles. Absorption oflight is caused by interaction with chromophores. From the reflectance(recovered light intensity) as a function of distance from the source,an SRS instrument can quantify the absorption and scatteringcoefficients of tissue. The absorption coefficient at two or morewavelengths can provide oxygenated and deoxygenated hemoglobinconcentrations and thereby tissue oxygen saturation within the volume oftissue interrogated. The wavelengths of the source(s) and the relativepositions of the source(s) with respect to the detectors allowmeasurements to be made for a predetermined tissue depth.

One field in which near-infrared spectroscopy, such as SRS, is useful isin tissue flap surgery in which a tissue flap is moved from one locationon a patient to another location for reconstructive surgery.Near-infrared spectroscopy techniques may be used to measure oxygensaturation in a tissue flap so that the viability of the tissue flap maybe determined in surgery and after surgery. Intraoperative tissue flapoximetry probes that employ near-infrared spectroscopy must be able toquickly deliver accurate oxygen saturation measurements under a varietyof non-ideal conditions. Current probes based on CWS have provensufficient for post-operative tissue monitoring where speed ofmeasurement is less critical and relative rather than absolutesaturation measurements are of concern. However, currently availableprobes have been shown to give inaccurate saturation measurements whenused intraoperatively due to common CWS assumptions. Embodiments of thepresently described invention provide improvements in tissue oximetryover known devices and techniques briefly described above.

Tissue Oximetry Device

FIG. 1A is a simplified image of a tissue oximetry device 100 that isself-contained according to one embodiment. Tissue oximetry device 100is configured to make tissue oximetry measurements, such asintraoperatively and postoperatively, without the need to communicate orinteract with other devices. In an implementation, the device ishandheld and can make tissue oximetry measurements and display thesemeasurements, without needing to connect to another external componenteither via a cable or wirelessly. The electronics to make measurementsand calculations is contained entirely within the housing of thehandheld device. The device is a standalone handheld tissue oximeterprobe, without a cable or wireless connection.

Tissue oximetry device 100 may be a handheld device that includes atissue oximetry probe 115 (also referred to as a sensor head), which maybe positioned at an end of a handle or sensing arm 114. Tissue oximetrydevice 100 is configured to measure the oxygen saturation of tissue byemitting light, such as near-infrared light, from tissue oximetry probe115 into tissue, and collecting light reflected from the tissue at thetissue oximetry probe.

Tissue oximetry device 100 may include a display 112 or othernotification device (e.g., a speaker) that notifies a user of oxygensaturation measured by the tissue oximetry device. While tissue oximetryprobe 115 is described as being configured for use with tissue oximetrydevice 100, which is a handheld device, tissue oximetry probe 115 may beused with other tissue oximetry devices, such as a modular tissueoximetry device where the tissue oximetry probe is at the end of a cabledevice that couples to a base unit. The cable device might be adisposable device that is configured for use with one patient and thebase unit might be a device that is configured for repeated use. Suchmodular tissue oximetry devices are well understood by those of skill inthe art and are not described further.

FIG. 1B is a block diagram of tissue oximetry device 100 according toone embodiment. Tissue oximetry device 100 includes display 112, aprocessor 116, a memory 117, a speaker 118, one or more user-selectiondevices 119 (e.g., one or more switches), a set of light sources 120, aset of detectors 125, and a power source (e.g., a battery) 127. Theforegoing listed components may be linked together via a bus 128, whichmay be the system bus architecture of tissue oximetry device 100.Although this figure shows one bus that connects to each component, thebusing is illustrative of any interconnection scheme serving to linkthese components or other components included in tissue oximetry device100 subsystems. For example, speaker 118 could be connected to asubsystem through a port or have an internal direct connection toprocessor 116. Further, the components described are housed in a mobilehousing (see FIG. 1A) of tissue oximetry device 100 according to atleast one embodiment.

Processor 116 may include a microprocessor, a microcontroller, controllogic, a multi-core processor, or the like. Memory 117 may include avariety of memories, such as a volatile memory 117 a (e.g., a RAM), anonvolatile memory 117 b (e.g., a disk, PROM, etc.). Differentimplementations of tissue oximetry device 100 may include any number ofthe listed components, in any combination or configuration, and may alsoinclude other components not shown.

Power source 127 can be a battery, such as a disposable battery.Disposable batteries are discarded after their stored charge isexpended. Some disposable battery chemistry technologies includealkaline, zinc carbon, or silver oxide. The battery has sufficientstored charged to allow use of the handheld device for several hours.After use, the handheld unit is discarded.

In other implementations, the battery can also be rechargeable where thebattery can be recharged multiple times after the stored charge isexpended. Some rechargeable battery chemistry technologies includenickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion(Li-ion), and zinc air. The battery can be recharged, for example, viaan AC adapter with cord that connects to the handheld unit. Thecircuitry in the handheld unit can include a recharger circuit (notshown). Batteries with rechargeable battery chemistry may be sometimesused as disposable batteries, where the batteries are not recharged butdisposed of after use.

FIGS. 2A and 2B are simplified end views of tissue oximetry probe 115according to one embodiment. The end views are the same but are markeddifferently for clarification. Tissue oximetry probe 115 is configuredto contact tissue (e.g., a patient's skin) for which a tissue oximetrymeasurement is to be made. Tissue oximetry probe 115 includes the set oflight sources 120 and includes the set of detectors 125. The set oflight sources 120 may include two or more light sources. According to aspecific implementation, tissue oximetry probe 115 includes three lightsources 120 a, 120 b, and 120 c, but according to other specificimplementations includes two light sources, such as light sources 120 aand 120 c. The specific implementation of tissue oximetry probe 115shown in FIGS. 2A and 2B includes the three light sources, 120 a, 120 b,and 120 c whereas the specific implementation of tissue oximetry probe115 shown in FIG. 2C includes fewer light sources. Specifically, tissueoximetry probe 115 shown in FIG. 2C has two light sources 120 a and 120c, where light source 120 b is omitted. Additional light sources (notshown) can be added.

Light sources 120 may be linearly positioned across tissue oximetryprobe 115 and detectors 125 may be arranged in an arc or a circle (i.e.,circular arrangement) on tissue oximetry probe 115. More specifically,light sources 120 may be arranged linearly, such as on a line (e.g., adiameter) that bisects a circle on which detectors 125 may be arranged.The outer light sources 120 a and 120 c are spaced a distance D1 apartwhere D1 may range from about 3 millimeters to about 10 millimeters. Thecentral light source 120 b may be positioned at an approximate mid pointbetween outer light sources 120 a and 120 c and is substantiallyequidistantly (+/−10 microns) from each detector 125 where the distancebetween the central light source and each detector is about 1.5millimeters to 5 millimeters. That is, the circle on which detectors 125are arranged may have a diameter of about 3 millimeters to about 10millimeters (e.g., 4 millimeters according to one specific embodiment).This maximum distance between the light sources and the detectors limitsreflectance data to light that propagated within the top layer of tissuewherein little or no underlying subcutaneous fat or muscular layersprior contributes to the reflectance data generated by detectors 125from light reflected from tissue. Propagation depth increases withincreasing source-to-detector distance, with about 4-5 millimetersgenerally being a sufficient upper limit to ensure few detected photonspropagated in lower tissue layers.

While detectors 125 are described as being arranged in an arc or circle,tissue oximetry device 100 may have other configurations of detectors,such as linear, square, rectangular, pseudo-random, or other arbitrarypattern.

The set of detectors 125 may include four or more detectors. Accordingto a specific embodiment, the set of detectors 125 includes eightdetectors 125 a, 125 b, 125 c, 125 d, 125 e, 125 f, 125 g, and 125 h asshown. Detectors 125 are solid state detectors and may be mounted to aPCB (not shown in FIGS. 2A-2C). Further, detectors 125 may be combineddevices or discrete devices. Processor 116 is configured to controllight sources 120 and detectors 125 via a set of electrical traces thatrun through the PCB. The circular configuration of detectors 125 and thelinear arrangement of light sources 125 allows for a relatively simplearrangement of the electrical traces. For example, the electrical tracesmay radially extend outward from lights sources 120 and detectors 125 sothat the electrical traces do not overlap in the PCB, which allows forrelatively even spacing between the electrical traces and therebyprovides for relatively low electrical crosstalk between the electricaltraces. In some situations, relatively low crosstalk between theelectrical traces lowers the signal-to-noise ratio of both the lightsources 120 and the detectors 125 as compared to electrical traces thatare alternatively arranged.

Detector Geometry for Increased Number of Data Points

In a specific implementation, detectors 125 are positioned with respectto outer light sources 120 a and 120 c such that four or more (e.g.,fourteen) unique source-to-detector distances are created. With greaternumbers of source-to-detector distances, this can be used to obtaingreater accuracy, faster calibration, and redundancy (when duplicatesource-to-detector distances are provided). At least twosource-to-detectors distances are about 1.5 millimeters or less (e.g.,0.5 millimeters up to about 1.7 millimeters), and at least two more twosource-to-detectors distances are about 2.5 millimeters or greater(e.g., 1.5 millimeters up to about 3.2 millimeters).

In other words, a first source-to-detector distance is about 1.5millimeters or less. A second source-to-detector distance is about 1.5millimeters or less. A third source-to-detector distance is about 2.5millimeters or greater. A fourth source-to-detector distance is about2.5 millimeters or greater. There can be various numbers of sources anddetector arrangements to obtain these four source-to-detector distances,such as one source and four detectors, two sources and two detectors,one detector and four sources, or other arrangements and combinations.

For example, an implementation includes at least two sources and atleast two detectors, where a maximum distance between a source and adetector is about 4 millimeters (or about 5 millimeters). At least twosource-to-detector are about 2.5 millimeters or greater. At least twosource-to-detector distances are about 1.5 millimeters or less.

When a greater number of sources and detectors are used, greater numbersof source-to-detector distances are available. As discussed, these canbe used to provide greater accuracy, faster calibration, or redundancy,or a combination thereof. The arrangement of the sources and detectorscan be in circular pattern, such as at points along the arc of a circlewith radius (e.g., 4 millimeters, or 5 millimeters). In animplementation, a tolerance of the detector or source positions on thearc is within 10 microns of the arc curve. In other implementations, thetolerance is within about 0.25 millimeters.

The foregoing described source-to-detectors distances allow for thedetermination of the scattering coefficient and the absorptioncoefficient from the reflectance data generated by detectors 125.Specifically, reflectance data generated for detectors having relativelysmall source-to-detector distances (e.g., 1.5 millimeters or closer) isa function of the scattering coefficient of tissue and not theabsorption coefficient, and reflectance data generated for detectorshaving relatively large source-to-detector distanced (e.g., 2.5millimeters or farther) is a function of the μ_(eff) (inverse of thepenetration depth), which is a function of both the scatteringcoefficient and the absorption coefficient. With at least two detectors125 positioned at 1.5 millimeters or closer to at least one light source120, and with at least two detectors positioned at 2.5 millimeters orfarther from at least one light source 120, the scattering coefficientand the absorption coefficient may be independently determined.

According to one specific embodiment, sixteen unique source-to-detectordistances are provided. The sixteen unique source-to-detector distancesmight be: 120 a-125 d=1.000 millimeters; 120 c-125 h=1.249 millimeters;120 a-125 e=1.500 millimeters; 120 c-125 a=1.744 millimeters; 120 a-125c=2.000 millimeters; 120 c-125 g=2.261 millimeters; 120 a-125 f=2.500millimeters; 120 c-125 b=2.712 millimeters; 120 a-125 b=2.940millimeters; 120 c-125 f=3.122 millimeters; 120 a-125 g=3.300millimeters; 120 c-125 c=3.464 millimeters; 120 a-125 a=3.600millimeters; 120 c-125 e=3.708 millimeters; 120 a-125 h=3.800millimeters; and 120 c-125 d=3.873 millimeters where these distances mayvary by about +/−10 microns.

In one alternative embodiment, the at least two source-to-detectordistances are the same, such as the shortest source-to-detectordistances. For example, the shortest source-to-detector distance D2between light source 120 a and detector 125 e, and the shortestsource-to-detector distance D3 between light source 120 c and detector125 a may be the same. It follows that the source-to-detector distanceD4 between light source 120 a and detector 125 a, and thesource-to-detector distance D5 between light source 120 c and detector125 e may also be the same. The source-to-detector distances D4 and D5are the longest source-to-detector distance for light sources 120 a and120 c. The foregoing description is for an example embodiment. Forexample, other pairs of source-to-detector distances might be the same,such as the next to shortest source-to-detector distances, and the nextto longest source-to-detector distances.

With the exception of the shortest source-to-detector distance and thelongest source-to-detector distance for light sources 120 a and 120 c,the source-to-detector distances for light sources 120 a and 120 c maybe unique. As described above, tissue oximetry probe 115 may havefourteen unique source-to-detector distances that allow for fourteendata points to be collected by detectors 125 from light emitted fromlight sources 120 a and 120 c.

Furthermore, the source-to-detector distances for light sources 120 aand 120 c may also be selected such that the increases in the distancesare substantially uniform. Thereby, a plot of source-to-detectordistance verses reflectance detected by detectors 125 can provide areflectance curve where the data points are substantially evenly spacedalong the x-axis. These spacings of the distances between light sources120 a and 120 c, and detectors 125 reduces data redundancy and can leadto the generation of relatively accurate reflectance curves.

Each light source 120 may include one or more light emitting diodes(LEDs), one or more laser diodes, one or more fiber optic cables, or acombination thereof. For example, each light source may include three orfour LEDs 130 that are coupled to a printed circuit board (PCB, notshown in FIGS. 2A and 2B) that routes control signals to the LEDs. TheLEDs included in one of the light sources 120 may generate and emitdifferent wavelengths and the LEDs included in the respective lightsources 120 may generate and emit the same sets of wavelengths. Forexample, the LEDs in light source 120 a may generate and emitwavelengths of approximately 760 nanometers (e.g., +/−10 nanometers),810 nanometers (e.g., +/−10 nanometers), and 850 nanometers (e.g., +/−20nanometers), and the LEDs respectively included in light sources 120 b,and 120 c may each generate and emit these three wavelengths.

FIGS. 3A and 3B are, respectively, a simplified perspective view and asimplified cross-sectional view of tissue oximetry probe 115 accordingto one specific embodiment. According to the embodiment shown in FIGS.3A and 3B, light sources 120 a, 120 b, and 120 c, respectively, includesets of fiber optic cables 135 a, 135 b, and 135 c (collectively fiberoptic cables 135) and include a number of LEDs 130. Each set of fiberoptic cables may include one or more fiber optic cables. According to anembodiment where each set of fiber optic cables includes more than onefiber optic cable, the fiber optic cables may be relatively narrow. LEDs130 may be mounted on a back PCB 150 and each of the fiber optic cables135 may be optically coupled to one or more of the LEDs to receive lightfrom the LEDs and emit light from tissue oximetry device 100. Forexample, according to an embodiment where each light source 120 includesthree LEDs 130 and one fiber optic cable 135, the fiber optic cable maybe optically coupled to the three LEDs included in the light source andmay receive light generated by the three LEDs for transmissions fromtissue oximetry device 100.

Detectors 125 may be mounted on the back PCB 150 or may be mounted on afront PCB 155. FIG. 4 is a simplified diagram of front PCB 155 showingdetectors 125 a-125 h positioned in a circular arrangement on this PCB.As described above, the circular arrangement of detectors 125 allows forthe electrical traces in PCB 155 or 150 to be routed in a relativelystraightforward configuration. The traces can radiate outward from thedetectors. This will minimize any crosstalk between the signals, sincethe interconnections do not have to cross over each other. The PCB canhave fewer layers. Thus, this design reduces complexity, improvesmanufacturability and yield, reduces parasitics in the signal path, andreduces cost. The design supports the use of discrete components, suchas discrete detectors 125 as compared to, for example, two or moredetectors 125 integrated in a single unit or package. Discrete detectorscan be less expensive to use or easier to source, or both. A circulararrangement of discrete detectors allows for a relatively large numberof unique source-detector positions in a relatively compact space of aPCB.

If detectors 125 are mounted to back PCB 150, fiber optic cables (notshown) may optically couple a front section 160 of tissue oximetry probe115 to the detectors where the fiber optic cables route detected lightto the detectors. Front section 160 of tissue oximetry probe 115 mayhave a number of apertures formed therein for light to pass from sources120 into tissue and to pass light reflected from the tissue to detectors125. PCBs 150 and 155 may include one or more of a variety of connectors(e.g., edge connectors) that electrically couple the PCBs to otherelectronic circuits (e.g., a processor, a memory, the display, andothers) in tissue oximetry device 100. While FIGS. 3A and 3B show anexample embodiment of tissue oximetry probe 115 including three lightsources 120 a, 120 b, and 120 c, the tissue oximetry probe may includefewer light sources (e.g., 120 a and 120 c) or more light sources.

FIGS. 5A and 5B are a simplified perspective view and an exploded view,respectively, of a tissue oximetry probe 115′ according to anotherspecific embodiment. The same numeral schema used for tissue oximetryprobe 115 is used to identify the same or similar elements of tissueoximetry probe 115′. Tissue oximetry probe 115′ is substantially similarto tissue oximetry probe 115 in that tissue oximetry probe 115′ includesouter light sources 120 a and 120 c and the set of detectors 125 whereouter light sources 120 a and 120 c and the set of detectors 125 havethe same positions as in tissue oximetry probe 115. Tissue oximetryprobe 115′ differs from tissue oximetry probe 115 in that tissueoximetry probe 115′ does not include central light source 120 b.

Outer light sources 120 a and 120 b include one or more LEDs or laserdiodes 130 (e.g., three LEDs) located on a back PCB 500. According to anembodiment where each outer light source includes three LEDs, the LEDsmay emit wavelengths of approximately 760 nanometers, 810 nanometers,and 850 nanometers. Detectors 125 are located on a front PCB 505. PCBs500 and 505 may include electrical traces for routing control signals tothe outer sources and the detectors.

Two sets of lenses 510 and 515 may be positioned, respectively, overouter light sources 120 a and 120 c to direct light emitted from theselight sources forward. More specifically, each set of lenses 510 and 515may include one or more lenses to direct light emitted from outer lightsources 120 a and 120 c forward. According to one specific embodiment,LEDs 130 are optically coupled to the lenses in a one-to-one mannerwhere each lens directs the light emitted from one LED forward. Thelenses may be hemispherical or the like. According to an alternativespecific embodiment, a single lens directs the light from LEDs 130forward.

Tissue oximetry probe 115′ may include a lens plate 520 that holds thelenses in alignment for optimal forward direction of emitted light. Lensplate 520 may be connected to an LED aperture plate 525 that has one ormore apertures formed therein for allowing light emitted from the outerlight sources 120 a and 120 c to pass forward to the sets of lenses 510and 515. Lens plate 520 may be connected to the back of front PCB 505,which may also have a number of apertures formed therein for allowingemitted light to pass forward. A contact plate 530 may be coupled to thefront of front PCB 505 and may also have apertures formed therein forallowing emitted light to pass forward from tissue oximetry device 100,and to allow light reflected from tissue to pass to detectors 125.

Calibration of Sources and Detectors

FIG. 6 is a high-level flow diagram of a method for calibrating eachsource-detector pair according to one embodiment. The high-level flowdiagram represents one example embodiment. Steps may be added to,removed from, or combined in the high-level flow diagram withoutdeviating from the scope of the embodiment.

At 600, tissue oximetry probe 115 contacts a tissue phantom, which hashomogeneous optical properties. Light (e.g., near-infrared light) isemitted from one or more of the light sources (e.g., outer light source120 a), step 605, into the tissue phantom and at least some of the lightis reflected back by the tissue phantom. Each detector 125 receives aportion of the light reflected from the tissue phantom, step 610, andeach detector generates reflectance data (i.e., a response) for theportion of reflected light received, step 615. The reflectance data fordetectors 125 may not match a reflectance curve for the tissue phantom(i.e., may be offset from the reflectance curve). If the reflectancedata generated by detectors 125 does not match the reflectance curve forthe tissue phantom, the detectors may have an intrinsic gain or loss.The reflectance data generated is used by tissue oximetry device 100 togenerate a set of calibration functions so that the raw reflectance datamatches the reflectance curve for the tissue phantom, step 620. Rawreflectance data includes the reflectance data generated and output bythe detectors prior to being utilized for determining the opticalproperties for the tissue and before being utilized for determiningoxygen saturation for the tissue.

Steps 600 to 620 may be repeated for one or more tissue phantoms. Thecalibration function for each source-detector pair for each tissuephantom should generally be the same. However, if there is a deviationbetween the calibration functions for a given source-detector pair for anumber of tissue phantoms, then the factors within the calibrationfunction for the given source-detector might be averaged. Each of thecalibration functions generated (including averaged functions) arestored in memory (e.g., Flash or other nonvolatile memory, or aprogrammable ROM), step 625.

Steps 600 to 625 may be repeated for each of the light sources, such aslight source 120 c. If steps 600 to 625 are repeated for light sources120 a and 120 c, for example, then two calibration functions may bestored in memory for each detector, and each of the stored calibrationfunctions for each detector is associated with one of the light sources.That is, each source-detector pair has a calibration functionspecifically for the source-detector pair. For example, detector 125 amay have a first calibration function stored for light emitted fromlight source 120 a (source-detector pair 125 a-120 a) and a secondcalibration function for light emitted from light source 120 c(source-detector pair 125 a-120 c). Because a calibration function isstored for each source-detector pair, the calibration functions (e.g.,two calibration functions) for each detector provide calibration notonly for variations in the detectors but also for variations in thelight sources. For example, the intrinsic gain or loss for a detectorshould not vary when receiving light from light source 120 a or 120 c.If the two calibration functions differ for a detector when receivingreflected light for light source 120 a and thereafter for 120 c, thedifference in the reflectance data for a given tissue phantom isattributable to differences in the intensity of light emitted by lightsource 120 a and 120 c. The calibration functions may be applied toreflectance data that is generated by detectors 125 when tissue oximetrydevice 100 is used for oxygen saturation measurement in real tissue, forexample, so that any intrinsic gains or losses of the detectors 125, andany difference in the intensity of light from light sources 125, may becompensated for. Specifically, the calibration functions are applied ona source-detector pair basis for the raw reflectance data generated bythe detectors.

As described briefly above, central light source 120 b may besubstantially equidistant (+/−10 microns) from each of detectors 125such that detectors 125 can be relatively easily calibrated usinghomogeneous tissue phantoms. The term “homogeneity” used with respect toa tissue phantom refers to the optical properties of a tissue phantombeing substantially constant throughout the volume of the tissuephantom. For example, the absorption coefficient μ_(a) and the reducedscattering coefficient μ_(s)′ of a tissue phantom may be referred to asbeing homogeneous (i.e., substantially constant) throughout the tissuephantom. This is in contrast to real tissue, which exhibits anisotropicoptical properties stemming from the intrinsic alignment of collagenfibers and other biological factors as well as the spatial variances,which may stem from differing degrees of tissue components and oxygensaturation.

FIG. 7 is a high-level flow diagram of a method for calibratingdetectors 125 according to one embodiment. The high-level flow diagramrepresents one example embodiment. Steps may be added to, removed from,or combined in the high-level flow diagram without deviating from thescope of the embodiment.

At 700, tissue oximetry probe 115 contacts a tissue phantom, which hashomogeneous optical properties. Light (e.g., near-infrared light) isemitted from central light source 120 b, step 705, into the tissuephantom and at least some of the light is reflected back by the tissuephantom. Each detector 125 receives the light reflected from the tissuephantom, step 710, and each detector generates a response to thereflected light, step 715. Each detector 125 should receive the sameamount of reflected light due to the homogeneity of the tissue phantom.Any differences between detector responses can therefore be attributedto physical differences between the detectors. For example, one or moreof the detectors may have an intrinsic gain or an intrinsic loss. Theresponses from detectors 125 are used by tissue oximetry device 100 togenerate calibration functions for the detectors, where the calibrationfunctions may be used by the tissue oximetry device to flatten the rawreflectance data (i.e., the responses) generated by the detectors to asingle value, step 720. The calibration functions or the responses, orboth, used for generating the calibration functions may be saved, e.g.,in a local memory (e.g., a Flash or nonvolatile memory, or programmableROM), step 725. The calibration functions may be applied to the rawreflectance data that are generated by detectors 125 when tissueoximetry device 100 is used to measure oxygen saturation levels in realtissue so that any intrinsic gains or losses of the detectors 125 may becompensated for.

FIG. 8 is a high-level flow diagram of a method for detecting anomaliesduring use of tissue oximetry device 100 according to one embodiment.The high-level flow diagram represents one example embodiment. Steps maybe added to, removed from, or combined in the high-level flow diagramwithout deviating from the scope of the embodiment.

Tissue oximetry device 100 may employ the method to detect anomaliessuch as significant, spatially congruous inhomogeneities in real tissue.Such an inhomogeneity can indicate the presence of a mole or type oftissue that does not contribute relevant information regarding theoxygenated hemoglobin and deoxygenated hemoglobin concentrations in atissue flap, for example. The inhomogeneity could also indicate thatpart of the probe has gone beyond the edge of a wound or is covered byblood.

At 800, light (e.g., near-infrared light) is emitted from central lightsource 120 b into tissue, and the light is reflected by the tissue intoone or more of detectors 125, step 805. Each detector 125 generates adetector response to the received light, step 810. If one or moredetectors lose contact with the tissue, then these detectors maygenerate a detector response, but the detector response might not be tolight emitted from central light source 120 b. Tissue oximetry device100 may determine whether the difference in the light detected (i.e.,detector response) by at least one of the detectors differs by athreshold amount compared to light detected by one or more of the otherdetectors, step 815.

If the detector responses to light emitted from central light source 120b differ between the detectors by the threshold amount (i.e., to agreater degree than predicted by ordinary tissue anisotropy), then thedetector responses from the at least one detector in the clear minorityof detector responses (i.e., detector response differs by at least thethreshold amount) may be discarded, step 820, and not used to calculateoxygen hemoglobin and deoxygenated hemoglobin concentrations. The atleast one detector in the clear minority may be assumed to have beenpositioned in contact with a mole, blood, or other or to have lostcontact with the tissue.

According to one alternative, if the detector responses generated by asignificant number (e.g., four) of detectors 125 differ significantly(e.g., by the threshold amount) from one another but there is no clearmajority of detector responses, then tissue oximetry device 100 maydisregard all of the detector responses and may indicate (e.g., ondisplay 112) that accurate oxygen saturation cannot be determined forthat currently probed region of tissue. The steps of the method may berepeated substantially continuously as tissue oximetry device 100measures oxygen saturation in tissue. It is noted that central source120 b might not otherwise be used for obtaining contributive data for areflectance curve used for determining oxygen saturation.

Self-Correction of Data During Oxygen Saturation Detection

FIG. 9 is a high-level flow diagram of a method for calibrating theamount of light emitted by outer sources 120 a and 120 c during oxygensaturation measurements on tissue or with a tissue phantom. Thehigh-level flow diagram represents one example embodiment. Steps may beadded to, removed from, or combined in the high-level flow diagramwithout deviating from the scope of the embodiment.

As described above, the shortest source-to-detector distances D2 and D3are intentionally matched for the two outer sources 120 a and 120 c andthe longest source-to-detector distances D4 and D5 are alsointentionally matched for the two outer sources. With the shortestsource-to-detector distances matched, when outer source 120 a emitslight, step 800, of a given wavelength into tissue and detector 125 edetects this light reflected from the tissue, step 905, and when outersource 120 c emits light into the tissue, step 910, and detector 125 adetects this light reflected from the tissue, step 815, the reflectancedata generated by detectors 125 a and 125 e, steps 920 and 925,respectively, should substantially match. That is, the amount of lightdetected by detectors 125 a and 125 e should substantially match.

Further, with the longest source-to-detector distances matched, whenouter source 120 a emits light of a given wavelength into tissue anddetector 125 a detects this light reflected from the tissue, and whenouter source 120 c emits light into the tissue and detector 125 edetects this light reflected from the tissue, the reflectance datagenerated by detectors 125 a and 125 e should also substantially match.If these pairs of reflectance data do not match, then the source powerof outer sources 120 a and 120 c and the amount of light emitted bythese outer sources may also be mismatched.

According to one embodiment, the tissue oximetry device uses these pairsof reflectance data (if mismatched) generated by detectors 125 a and 125e to correct the reflectance data generated by all of the detectors andto correct the oxygen saturation analysis performed by the device. Morespecifically, a calibration function, step 930, for the reflectance data(due to a source power difference between outer sources 120 a and 120 c)can be determined from the difference between the absolute reflectancedetected by detectors 125 a and 125 e. This calibration function can beapplied to the raw reflectance data generated by each detector 125 tocompensate for the difference in the amount of light emitted by outersources 120 a and 120 c. Specifically, two sets of reflectance datapoints that are offset from each other can be brought onto a singlereflectance curve by applying the generated function to the reflectancedata generated by each detector 125 thereby generating relatively moreaccurate oxygen saturation data.

Tissue oximetry device 100 may substantially continuously monitor andcompare the reflectance data generated by detectors 125 a and 125 e todetermine whether differences in the amount of light emitted from thetwo outer sources 120 a and 120 c occurs. Using the differences (ifpresent), the reflectance data for each of detectors 125 can besubstantially continuously corrected by tissue oximetry device 100during oxygen saturation measurements. According to one alternativeembodiment, the calibration of the outer sources is performed once andthe generated function is stored for later use while making oxygensaturation measurements.

According to one alternative, additional or alternativesource-to-detector distances can be matched for generating a functionfor the reflectance data due to source power difference between outersources 120 a and 120 c (i.e., calibrating outer sources 120 a and 120c). That is, the shortest or longest source-to-detector distances (or acombination of these) are not required for calibrating outer sources 120a and 120 c and for correcting the reflectance data. Furthermore, whileusing two or more pairs of matched source-to-detectors distances mayincrease the reliability or accuracy of the source calibration, a singlematched pair of source-to-detector distances may be used for calibratingouter sources 120 a and 120 c.

If a single matched pair of source-to-detector distances (e.g., D2 andD3) is used to calibrate outer sources 120 a and 120 c and forcorrecting the reflectance data, then the signal-to-noise ratio of thereflectance data may be relevant for selecting the particularsource-to-detector distance to match. If minimal to low noise ispresent, then matching the longest source-to-detector distances mayprovide the most robust source calibration. However, noise may increaseas the square root of the magnitude of a reflectance data measurement,and therefore may be significantly larger for longer source-to-detectordistances. In this case, matching the shortest or relatively shortsource-to-detector distances may provide a more robust calibration ofthe outer sources and the reflectance data.

According to another alternative, all of the source-to-detectordistances for the outer sources 120 a and 120 c, and the detectors 125a-125 h are matched providing four matched source-to-detector distances.Matching four source-to-detector distances for outer sources 120 a and120 c allows for the generation of two reflectance data sets for eachouter source, which can be compared to verify accuracy of the reflectiondata.

The geometrical incorporation of fast and robust calibration,self-correction, and accurate data collection and processing methodslimits fluctuations and inaccuracy seen in saturation measurements madeby the intra-operative probes considered to be prior art. The previouslydiscussed calibration, self-correction, and other features can lead to afast, accurate tissue oximetry devices, which should be desirable toplastic surgeons involved in implant-based breast reconstruction andothers concerned with detecting tissue regions in danger of necrosis insurgical environments.

This description of the invention has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise form described, and manymodifications and variations are possible in light of the teachingabove. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications.This description will enable others skilled in the art to best utilizeand practice the invention in various embodiments and with variousmodifications as are suited to a particular use. The scope of theinvention is defined by the following claims.

The invention claimed is:
 1. A method for operating a tissue oximetry device comprising: emitting light from a first light source of a plurality of light sources of the tissue oximetry device into a tissue phantom, wherein the emitted light is reflected by the tissue phantom; detecting the reflected light from the tissue phantom using a plurality of detectors of the tissue oximetry device, wherein the plurality of sources are arranged along a line, and there are an equal number of detectors on either side of the line, and a position of a detector on a first side of the line will have point symmetry with another detector on a second side of the line about a selected point on the line, and each detector above the line has a distance to the first light source that is different from every other detector above the line; generating a set of detector responses by the plurality of detectors based on detecting the light emitted from the light source; comparing the set of detector responses to a previously determined reflectance curve for the tissue phantom; generating a set of calibration functions based on the comparison of the set of detector responses to the previously determined reflectance curve, wherein each calibration function in the set of calibration functions is associated with a different pair of the first light source and each detector above the line, each different pair having a different source-to-detector separation distance; and storing the set of calibration functions in a memory of the tissue oximetry device.
 2. The method of claim 1 wherein the selected point on the line is a midpoint between the first light source and a second light source of the plurality of light sources.
 3. The method of claim 1 wherein a distance between a detector above the line to the first light source is less than about 4 millimeters.
 4. The method of claim 1 wherein the plurality of light sources comprises a second light source, and the first light source, second light source, and plurality of detectors are arranged in a circle.
 5. A tissue oximeter probe calibrated according to the method of claim
 1. 6. A method comprising: providing a tissue oximeter comprising a sensor head and a nonvolatile memory, wherein the sensor head comprises a plurality of sources and a plurality of detectors, the sources are arranged along a line, there are an equal number of detectors on either side of the line, and a position of a detector on a first side of the line will have point symmetry with another detector on a second side of the line about a selected point on the line, and each detector above the line has a distance from each source that is different from every other detector above the line; emitting light from a first source of a plurality of sources of the tissue oximetry device into a tissue phantom, wherein the emitted light is reflected by the tissue phantom; detecting the reflected light from the tissue phantom using the plurality of detectors of the tissue oximetry device; generating a set of detector responses by the plurality of detectors based on detecting the light emitted from the light source; comparing the set of detector responses to a previously determined reflectance curve for the tissue phantom; generating a set of calibration functions based on the comparison of the set of detector responses to the previously determined reflectance curve, wherein each calibration function in the set of calibration functions is associated with a different pair of a source of the plurality of sources and each detector above the line, each different pair having a different source-to-detector separation distance; and storing the set of calibration functions in the nonvolatile memory of the tissue oximetry device.
 7. The method of claim 6 wherein the selected point on the line is a midpoint between the first light source and a second light source of the plurality of light sources.
 8. The method of claim 6 wherein a distance between a detector above the line to the first light source is less than about 4 millimeters.
 9. The method of claim 6 wherein the plurality of light sources comprises a second light source, and the first light source, second light source, and plurality of detectors are arranged in a circle.
 10. The method of claim 6 wherein a first distance is from a first detector of the plurality of detectors to the first source, a second distance is from the first detector to the second source, and the first distance is greater than the second distance, a third distance is from a second detector of the plurality of detectors to the first source, a fourth distance is from the second detector to the second source, and the fourth distance is greater than the third distance, a fifth distance is from a third detector of the plurality of detectors to the first source, a sixth distance is from the third detector to the second source, the fifth distance is different from the first distance and the second distance, and the sixth distance is different from the first distance and the second distance, a seventh distance is from a fourth detector of the plurality of detectors to the first source, an eighth distance is from the fourth detector to the second source, the seventh distance is different from the first, second, and fifth distances, the eighth distance is different from the first, second, and sixth distances, and the first distance is greater than the fifth, sixth, seventh, and eighth distances, and the second distance is less than the fifth, sixth, seventh, and eighth distances.
 11. A method comprising: providing a tissue oximeter comprising a sensor head and a memory, wherein the sensor head comprises a plurality of sources and a plurality of detectors, the sources are arranged along a line, there are an equal number of detectors on either side of the line, and a position of a detector on a first side of the line will have point symmetry with another detector on a second side of the line about a selected point on the line, and each detector above the line has a distance from each source that is different from every other detector above the line; emitting light from a first source of a plurality of sources of the tissue oximetry device into a tissue phantom, wherein the emitted light is infrared light and reflected by the tissue phantom; detecting the reflected light from the tissue phantom using the plurality of detectors of the tissue oximetry device; generating a set of detector responses by the plurality of detectors based on detecting the light emitted from the light source; comparing the set of detector responses to a previously determined reflectance curve for the tissue phantom; generating a set of calibration functions based on the comparison of the set of detector responses to the previously determined reflectance curve, wherein each calibration function in the set of calibration functions is associated with a different pair of a source of the plurality of sources and each detector above the line, each different pair having a different source-to-detector separation distance; and storing the set of calibration functions in the memory of the tissue oximetry device.
 12. The method of claim 11 wherein the selected point on the line is a midpoint between the first light source and a second light source of the plurality of light sources.
 13. The method of claim 11 wherein a distance between a detector above the line to the first light source is less than about 4 millimeters.
 14. The method of claim 11 wherein the plurality of light sources comprises a second light source, and the first light source, second light source, and plurality of detectors are arranged in a circle.
 15. The method of claim 11 wherein a first distance is from a first detector of the plurality of detectors to the first source, a second distance is from the first detector to the second source, and the first distance is greater than the second distance, a third distance is from a second detector of the plurality of detectors to the first source, a fourth distance is from the second detector to the second source, and the fourth distance is greater than the third distance, a fifth distance is from a third detector of the plurality of detectors to the first source, a sixth distance is from the third detector to the second source, the fifth distance is different from the first distance and the second distance, and the sixth distance is different from the first distance and the second distance, a seventh distance is from a fourth detector of the plurality of detectors to the first source, an eighth distance is from the fourth detector to the second source, the seventh distance is different from the first, second, and fifth distances, the eighth distance is different from the first, second, and sixth distances, and the first distance is greater than the fifth, sixth, seventh, and eighth distances, and the second distance is less than the fifth, sixth, seventh, and eighth distances.
 16. A method comprising: providing a tissue oximeter comprising a sensor head and a memory, wherein the sensor head comprises a plurality of sources and a plurality of detectors, the sources are arranged along a line, there are an equal number of detectors on either side of the line, a position of a detector on a first side of the line will have point symmetry with another detector on a second side of the line about a selected point on the line, and the plurality of detectors comprises four detectors above the line and four detectors below the line, and each detector above the line has a distance from each source that is different from every other detector above the line; emitting light from a first source of a plurality of sources of the tissue oximetry device into a tissue phantom, wherein the emitted light is reflected by the tissue phantom; detecting the reflected light from the tissue phantom using the plurality of detectors of the tissue oximetry device; generating a set of detector responses by the plurality of detectors based on detecting the light emitted from the light source; comparing the set of detector responses to a previously determined reflectance curve for the tissue phantom; generating a set of calibration functions based on the comparison of the set of detector responses to the previously determined reflectance curve, wherein each calibration function in the set of calibration functions is associated with a different pair of a source of the plurality of sources and each detector above the line, each different pair having a different source-to-detector separation distance; and storing the set of calibration functions in the memory of the tissue oximetry device.
 17. The method of claim 16 wherein the selected point on the line is a midpoint between the first light source and a second light source of the plurality of light sources.
 18. The method of claim 16 wherein a distance between a detector above the line to the first light source is less than about 4 millimeters.
 19. The method of claim 16 wherein the plurality of light sources comprises a second light source, and the first light source, second light source, and plurality of detectors are arranged in a circle.
 20. The method of claim 16 wherein a first distance is from a first detector of the plurality of detectors to the first source, a second distance is from the first detector to the second source, and the first distance is greater than the second distance, a third distance is from a second detector of the plurality of detectors to the first source, a fourth distance is from the second detector to the second source, and the fourth distance is greater than the third distance, a fifth distance is from a third detector of the plurality of detectors to the first source, a sixth distance is from the third detector to the second source, the fifth distance is different from the first distance and the second distance, and the sixth distance is different from the first distance and the second distance, a seventh distance is from a fourth detector of the plurality of detectors to the first source, an eighth distance is from the fourth detector to the second source, the seventh distance is different from the first, second, and fifth distances, the eighth distance is different from the first, second, and sixth distances, and the first distance is greater than the fifth, sixth, seventh, and eighth distances, and the second distance is less than the fifth, sixth, seventh, and eighth distances.
 21. A method comprising: providing a tissue oximeter comprising a sensor head and a memory, wherein the sensor head comprises a plurality of sources and a plurality of detectors, the sources are arranged along a line, there are an equal number of detectors on either side of the line, and a position of a detector on a first side of the line will have point symmetry with another detector on a second side of the line about a selected point on the line, and each detector above the line has a distance from each source that is different from every other detector above the line; emitting light from a first source of a plurality of sources of the tissue oximetry device into a tissue phantom, wherein the emitted light is reflected by the tissue phantom; detecting the reflected light from the tissue phantom using the plurality of detectors of the tissue oximetry device; generating a set of detector responses by the plurality of detectors based on detecting the light emitted from the light source; comparing the set of detector responses to a previously determined reflectance curve for the tissue phantom; generating a set of calibration functions based on the comparison of the set of detector responses to the previously determined reflectance curve, wherein each calibration function in the set of calibration functions is associated with a different pair of a source of the plurality of sources and each detector above the line, each different pair having a different source-to-detector separation distance; and storing the set of calibration functions in the memory of the tissue oximetry device, wherein the tissue phantom is a first tissue phantom, and the method comprises repeating the emitting light, detecting the reflected light, generating a set of detector responses, comparing the set of detector responses, and generating a set of calibration functions for a second tissue phantom, which is different from the tissue phantom.
 22. The method of claim 21 wherein the selected point on the line is a midpoint between the first light source and a second light source of the plurality of light sources.
 23. The method of claim 21 wherein a distance between a detector above the line to the first light source is less than about 4 millimeters.
 24. The method of claim 21 wherein the plurality of light sources comprises a second light source, and the first light source, second light source, and plurality of detectors are arranged in a circle.
 25. The method of claim 21 wherein a first distance is from a first detector of the plurality of detectors to the first source, a second distance is from the first detector to the second source, and the first distance is greater than the second distance, a third distance is from a second detector of the plurality of detectors to the first source, a fourth distance is from the second detector to the second source, and the fourth distance is greater than the third distance, a fifth distance is from a third detector of the plurality of detectors to the first source, a sixth distance is from the third detector to the second source, the fifth distance is different from the first distance and the second distance, and the sixth distance is different from the first distance and the second distance, a seventh distance is from a fourth detector of the plurality of detectors to the first source, an eighth distance is from the fourth detector to the second source, the seventh distance is different from the first, second, and fifth distances, the eighth distance is different from the first, second, and sixth distances, and the first distance is greater than the fifth, sixth, seventh, and eighth distances, and the second distance is less than the fifth, sixth, seventh, and eighth distances.
 26. A method comprising: providing a tissue oximeter comprising a sensor head and a memory, wherein the sensor head comprises a plurality of sources and a plurality of detectors, the sources are arranged along a line, there are an equal number of detectors on either side of the line, and a position of a detector on a first side of the line will have point symmetry with another detector on a second side of the line about a selected point on the line, and each detector above the line has a distance from each source that is different from every other detector above the line; emitting light from a first source of a plurality of sources of the tissue oximetry device into a tissue phantom, wherein the emitted light is reflected by the tissue phantom; detecting the reflected light from the tissue phantom using the plurality of detectors of the tissue oximetry device; from the electrical signals, generating a set of detector responses by the plurality of detectors based on detecting the light emitted from the light source; comparing the set of detector responses to a previously determined reflectance curve for the tissue phantom; generating a set of calibration functions based on the comparison of the set of detector responses to the previously determined reflectance curve, wherein each calibration function in the set of calibration functions is associated with a different pair of a source of the plurality of sources and each detector above the line, each different pair having a different source-to-detector separation distance; and storing the set of calibration functions in the memory of the tissue oximetry device.
 27. The method of claim 26 wherein the selected point on the line is a midpoint between the first light source and a second light source of the plurality of light sources.
 28. The method of claim 26 wherein a distance between a detector above the line to the first light source is less than about 4 millimeters.
 29. The method of claim 26 wherein the plurality of light sources comprises a second light source, and the first light source, second light source, and plurality of detectors are arranged in a circle.
 30. The method of claim 26 wherein a first distance is from a first detector of the plurality of detectors to the first source, a second distance is from the first detector to the second source, and the first distance is greater than the second distance, a third distance is from a second detector of the plurality of detectors to the first source, a fourth distance is from the second detector to the second source, and the fourth distance is greater than the third distance, a fifth distance is from a third detector of the plurality of detectors to the first source, a sixth distance is from the third detector to the second source, the fifth distance is different from the first distance and the second distance, and the sixth distance is different from the first distance and the second distance, a seventh distance is from a fourth detector of the plurality of detectors to the first source, an eighth distance is from the fourth detector to the second source, the seventh distance is different from the first, second, and fifth distances, the eighth distance is different from the first, second, and sixth distances, and the first distance is greater than the fifth, sixth, seventh, and eighth distances, and the second distance is less than the fifth, sixth, seventh, and eighth distances.
 31. A method comprising: providing a tissue oximeter comprising a sensor head and a memory, wherein the sensor head comprises a plurality of sources and a plurality of detectors, the sources are arranged along a line, there are an equal number of detectors on either side of the line, and a position of a detector on a first side of the line will have point symmetry with another detector on a second side of the line about a selected point on the line, and each detector above the line has a distance from each source that is different from every other detector above the line; emitting light from a first source of a plurality of sources of the tissue oximetry device into a tissue phantom, wherein the emitted light is reflected by the tissue phantom comprises emitting light from a first source at a first wavelength tissue phantom, emitting light from a first source at a second wavelength tissue phantom, emitting light from a first source at a first wavelength tissue phantom, and emitting light from a first source at a first wavelength tissue phantom, wherein the first, second, third, and fourth wavelengths are different from each other; detecting the reflected light from the tissue phantom using the plurality of detectors of the tissue oximetry device; generating a set of detector responses by the plurality of detectors based on detecting the light emitted from the light source; comparing the set of detector responses to a previously determined reflectance curve for the tissue phantom; generating a set of calibration functions based on the comparison of the set of detector responses to the previously determined reflectance curve, wherein each calibration function in the set of calibration functions is associated with a different pair of a source of the plurality of sources and each detector above the line, each different pair having a different source-to-detector separation distance; and storing the set of calibration functions in the memory of the tissue oximetry device.
 32. The method of claim 31 wherein the selected point on the line is a midpoint between the first light source and a second light source of the plurality of light sources.
 33. The method of claim 31 wherein a distance between a detector above the line to the first light source is less than about 4 millimeters.
 34. The method of claim 31 wherein the plurality of light sources comprises a second light source, and the first light source, second light source, and plurality of detectors are arranged in a circle.
 35. The method of claim 31 wherein the tissue phantom is a first tissue phantom, and the method comprises repeating the emitting light at each of first, second, third, and fourth wavelengths for a second tissue phantom, which is different from the tissue phantom.
 36. The method of claim 31 wherein a first distance is from a first detector of the plurality of detectors to the first source, a second distance is from the first detector to the second source, and the first distance is greater than the second distance, a third distance is from a second detector of the plurality of detectors to the first source, a fourth distance is from the second detector to the second source, and the fourth distance is greater than the third distance, a fifth distance is from a third detector of the plurality of detectors to the first source, a sixth distance is from the third detector to the second source, the fifth distance is different from the first distance and the second distance, and the sixth distance is different from the first distance and the second distance, a seventh distance is from a fourth detector of the plurality of detectors to the first source, an eighth distance is from the fourth detector to the second source, the seventh distance is different from the first, second, and fifth distances, the eighth distance is different from the first, second, and sixth distances, and the first distance is greater than the fifth, sixth, seventh, and eighth distances, and the second distance is less than the fifth, sixth, seventh, and eighth distances. 